Hybrid metal packages have been used for many years to hermetically protect hybrid and semiconductor discrete and integrated circuit chips. The chips are mounted within the eyelet or body of the package and are electrically connected to external circuitry by means of conductive leads or pins passed through apertures in the body. To ensure hermetic sealing and to preclude short circuiting between the lead or pins and the metallic package the leads or pins are sealed in glass and the glass-pin combination is sealed in the body.
The formation of hermetic glass-to-metal seals, as exemplified in U.S. Pat. Nos. 3,370,874, 3,600,017 and 4,008,945, utilize glass and metal components possessing dissimilar coefficients of thermal expansion (CTE) so that as thermal energy is applied and removed during seal formation and during subsequent operational use the glass and metal components expand and contract, respectively, at different rates. Hermeticity in compression glass-to-metal seals is achieved, without bonding, by the compressive forces or physical stresses created between the components by the dissimilar rates of contraction and expansion.
Matched glass-to-metal seals, in contrast, are formed by utilizing glass and metal components possessing approximately equivalent CTEs so that as thermal energy is applied and removed during the sealing operation and during subsequent operational use the components expand and contract, respectively, at approximately the same rate. Commonly used materials for matched glass-to-metal seals are Kovar or F-15 metal alloy, an alloy containing approximately 17 percent cobalt, 29 percent nickel and 54 percent iron, and a borosilicate or "hard glass" such as Corning 7052. The CTEs of Kovar and hard glass are approximately equivalent, on the order of about 5.4.times.10.sup.-6 in/in/.sup.o C.
Hermeticity in matched glass-to-metal seals is achieved by molecular bonding between the glass and metal components. The surfaces of the metal components interfacing with the glass are preconditioned prior to the sealing operation by the controlled growth of munsel-grey oxide on the interfacing surfaces of the metallic components. The body/glass/pin combination is assembled and thermal energy is applied thereto sufficient to partly fluidize or melt the glass. The surface tension of the partially fluidized glass causes it to wet the musel-grey oxide of the interfacing surfaces of the metal components. As thermal energy is withdrawn molecular bonding between the glass and metal components occurs to form the glass-to-metal seals.
FIG. 1 exemplifies a matched glass-to-metal 10 seal of the prior art wherein a hard glass preform 12 is sealed to an F-15 alloy terminal pin or lead 14 along interfacing surface 16 and to an F-15 alloy body 18 along interfacing surface 20. Positive menisci 22, 24 are formed at the ends of the interfaces 16, 20, respectively, i.e., at the boundary of the interfaces with the environment.
A large percentage of hybrid metal packages find end use in both high-tech industrial and governmental applications. It is therefore more efficacious, as a practical matter, to subject hybrid metal packages to quality control (QC) acceptance testing using QC standards meeting or exceeding government QC standards, rather than segregating hybrid metal package lots according to end use and then QC testing using different criteria. Not only would the latter procedure increase the overall production time and cost, necessitating for example tighter package lot control and segregation and recalibration or duplication of QC gear, but it would also vitiate the fungibility of finished metal packages.
Hybrid metal packages are generally subjected to four broad areas of QC testing: visual/mechanical; electrical; environmental; and line. Hybrid packages subjected to mechanical forces such as acceleration, shock or vibration, either during QC testing or in field use, have been found to experience a certain degradation in physical integrity.
Due to the partial fluidization and resolidification of the boundary layers of glass at the interfaces 16, 20 and the environmentally-exposed surfaces, compressive skin stresses are set up in the outer glass layers. When subjected to mechanical forces, matched glass-to-metal seals of the prior art have been found to be deleteriously affected by crack or fracture formation and glass chip-out.
Of particular concern are the glass menisci 22, 24 formed during the sealing operation, these menisci being brittle and of relatively low toughness, strength and ductility. The menisci 22, 24 are subject to the highest concentration of compressive skin stresses due to their structural configuration. The menisci 22, formed at the interface 16 of the glass 12-terminal pin 14, is especially vulnerable since most of the mechanical forces experienced by the hybrid packages are transmitted by means of the terminal pins or leads 14.
The largest single cause of hybrid metal package failure results from spreading meniscus cracks which exceed fifty percent of the distance from the terminal pin 14 to the eyelet 18 and/or glass chip-out from the meniscus 22.
A typical propagation route 26 for meniscus cracks is shown graphically in FIG. 2. In general a crack, once formed in the meniscus 22, proceeds inwardly into the glass 12 for some distance, and then at some point hooks back towards the surface of the meniscus 22, such that the crack propagation route 26 is generally "fishhook" shaped. The crack propagation route 26 is shown generally as A-B-C-D-E-F-G. A glass chip-out is shown generally at 28 and represents a segment of glass lost from the glass-to-metal seal as a result of a completed fishhook crack.
Cracks or fractures formed in the meniscus, or chip-out losses from the meniscus, adversely affect the physical integrity of hybrid metal packages. Theoretically, all hermetic packages leak to a certain extent. A "hermetic" package is pragmatically defined as one having an acceptable leak rate, and for most applications the hermeticity is satisfactory if the leak rate is equal to or less than 1 X 10.sup.-8 cubic centimeters of helium per second at a pressure differential of one atmosphere. Cracks or chip-outs in the meniscus may cause the package to have a leak rate greater than 1.times.10.sup.-8 cc/He/sec.
Matched glass-to-metal seals are generally subjected to a salt atmosphere during QC environmental testing. The salt atmosphere will readily penetrate any chip-outs or cracks formed in the meniscus, and if the penetration is sufficient to contact the terminal pin or lead, a corroding action will be engendered thereon. A corroded pin or lead may eventually result in the degradation or complete failure of the hybrid package.
Various solutions have been propounded to reduce the complications arising from meniscus cracking or chip-out loss. These include pressing the glass flat in the meniscus area, treating the glass surface with buffered or unbuffered fluoride ions, undersealing to reduce the degree of meniscus wickup and fire polishing at a slightly lower temperature than the fluidizing temperature during sealing. These solutions not only increase the cost of matched glass-to-metal seal, but adversely affect the characteristics of the glass-to-metal seal.
Another approach involves capping the glass preform with a ceramic disk. The ceramic disk and the glass preform, however, have dissimilar CTEs such that when the ceramic capped glass preform is subjected to thermal shock or extremes in temperature cycling the ceramic disk and the glass preform expand and contract at different rates. The disparity in expansion and contraction leads to crack or fracture formation and/or separation along the interface between the ceramic disk and glass preform.